1. Field of the Invention
The present invention relates generally to a method of measuring the flow through porous media and an apparatus therefor, and more particularly to a method and apparatus for measuring the transport properties of a fluid flowing through porous materials using nuclear magnetic resonance (NMR).
2. Description of the Art
The study of flow through porous media provides a means of understanding the process of oil recovery and is therefore of considerable interest to the oil industry. NMR is well suited for the study of the behaviour of liquids in solid porous materials, as found in oil reservoirs for example.
Echo-planar imaging (EPI) (P. Mansfield, Jnl. Phys. C: Solid State Phys. 10 L55 (1977)) is a high speed NMR imaging method which can obtain a full 2D spatially resolved NMR image from one FID. Typically a 128.times.128 image is acquired in 100 ms. The technique has been applied successfully in a range of clinical studies (M. K. Stehling, A. M. Howseman et al, Radiology 170 257-263 (1989), A. M. Howseman, M. K. Stehling et al, Brit. Jnl. Radiol. 61 822-828 (1988), M. K. Stehling, R. M. Charnley, et al, Brit. Jnl. Radiol. 63 430-437 (1990)) and is especially valuable in parts of the anatomy where there is considerable involuntary motion.
Porous rock samples saturated with water have very large local field inhomogeneities caused by the susceptibility differences of the rock material itself and absorbed water. The largest inhomogeneities occur at the boundaries between liquid and solid where the magnetic susceptibility of the material changes dramatically. The effect of locally induced inhomogeneity is to shorten the effective transverse relaxation time T2*. This means that the resulting NMR signal is not coherent for a sufficiently long enough time period to perform a standard EPI experiment. This is because standard EPI uses gradient reversals to form a series of gradient echoes which are used to encode spatial information and there is no refocusing mechanism for short T2* components.
It is well known that NMR spin echoes produced by application of a 180.degree. radio frequency (RF) pulse rather than a gradient reversal are able to effectively remove local inhomogeneity. A modified EPI sequence can be used for the study of porous media where the gradient reversals are replaced by 180.degree. RF pulses. This means that the technique is limited only by true T2 and not the very short T2* caused by large local inhomogeneous field gradients. The modified sequence also maintains the high speed characteristics of EPI with typical exposure times around 100 ms and can be used for real time visualisation of the fluid dynamics inside the porous medium.
In order to achieve flow quantification a spin preparation procedure is required prior to the imaging sequence. Flow encoding procedures for medical applications are well documented (P. R. Moran, R. A. Moran and N. Karsteadt, Radiology, 154 433-411 (1985), P. R. Moran, Mag. Res. Imaging, 1 197-203 (1982)) and currently flow techniques are used to produce very high quality NMR angiograms (projective images) of the major blood vessels in the human body (C. L. Dumoulin, S. P. Souza and H. R. Hart, Mag. Res. Med. 5 238-345 (1987), D. G. Nishimura, Mg. Res. Med. 14 194-201 (1990)). The flow encoding procedure uses a bipolar gradient producing a phase difference between moving and static spins which is linearly dependent on the velocity of the moving spins in the direction of the applied flow encoding gradient.
However a flow encoding procedure for liquids inside porous media must simultaneously overcome the effects of both field inhomogeneity and translational diffusion. Diffusion in the presence of a gradient causes irreversible loss of signal. In the case of liquids in porous media the diffusion process causes a signal attenuation from the applied flow encoding and imaging gradients as well as from the locally induced field gradients. The flow encoding procedure must therefore minimise these effects so as to ensure that there is sufficient signal for imaging after flow preparation.